Low Emission Engines for Heavy-Duty Natural Gas-Powered Urban Vehicles - Development Experience

The evolution and explanation of an approach to achieving a stated set of very low emissions limits was described in a previous paper (1)*. The method outlined was to use stoichiometric mixture preparation with EGR dilution in order to employ a 3-way catalyst for low emissions, whilst giving an engine power output competitive with a turbocharged diesel engine.

This approach has been followed in an engine development programme, which has resulted in a responsive and driveable engine being produced. The engine has demonstrated the achievability of very low emissions over the US heavy duty diesel transient test (FTP) cycle as follows:

The lean-burn approach to low emission heavy duty operation has also been considered, using steady-state engine test results. The NOx-HC trade-off has been identified as a key indicator of engines' potential, and is also considered to give an indication of the accuracy of air-fuel ratio control required to achieve proposed emissions standards.

Results from stoichiometric, naturally aspirated gas engines, indicate that they have potential for low emission operation using 3-way catalysts. Although considered unsuitable for heavy-duty application due to knock and temperature limitations, this type of engine could find use in light-medium duty applications, where greater torque and power then diesel engines and better efficiencies than gasoline engines would be attractive features.